The use of renewable energy sources to generate electricity for the power network is becoming increasingly common in many countries. It is possible to convert renewable energy such as wind, wave, tidal energy or water current flows into electrical energy by using a turbine to drive the rotor of an alternating current (ac) electrical power generator, either directly or by means of a gearbox. The ac frequency that is developed at the stator terminals of the generator is directly proportional to the speed of rotation of the rotor. The voltage at the generator terminals also varies as a function of speed and, depending on the particular type of generator, on the flux level.
In some circumstances, it can be advantageous to transmit electrical power generated by a renewable energy turbine via a high-voltage direct current (HVDC) power transmission network, as opposed to a more conventional ac power transmission network. A dc source power converter in the form of a generator bridge and operating as an active rectifier connects the ac electrical power generator of the renewable energy turbine to the HVDC power transmission network. The renewable energy turbine and its associated ac electrical power generator and dc source power converter thus operate together as an individual dc source supplying dc electrical power to the HVDC power transmission network. It will be understood that a large number of such dc sources are typically connected in parallel to the HVDC power transmission network to supply the required amount of dc electrical power to the network and ensure stable network operation.
The individual dc sources can operate under voltage control regulation to supply electrical power at a target or reference voltage value Vref to the HVDC power transmission network and/or under current control regulation to supply electrical power at a target or reference current value Iref to the HVDC power transmission network, with a combination of voltage control regulation and current control regulation being more advantageous. When a dc source power converter is used to connect an ac electrical power generator of a renewable energy turbine to a HVDC power transmission network, the use of current control regulation alone has been adopted as it is simple to implement and inherently stable.
During fault conditions, either in the HVDC power transmission network or in one or more of the parallel-connected dc sources, the output voltage at the converter terminals of one or more of the individual dc source power converters or the output current supplied by one or more of the individual dc source power converters can increase to levels that cannot be tolerated by the dc system. For example, when the dc source is operating under current control regulation, the reference voltage value Vref of the dc source is determined by the HVDC power transmission network and, more particularly, other devices connected to the HVDC power transmission network in parallel with the dc source. However, if the dc source becomes disconnected from the HVDC power transmission network during fault conditions, the dc source will lose its reference voltage value Vref but will continue to supply power at the same reference current value Iref. This may lead to a dangerously high output voltage at the converter terminals of the dc source power converter.
There is, therefore, a need for a converter control arrangement and associated control methodology for a dc source power converter which is capable of providing reliable and effective regulation of the output current of a dc source power converter to minimise damage, either arising from excessive output voltage at the converter terminals or from excessive output current, especially during fault conditions.